Environmental legislation demands constant improvements in the cleanliness of engine exhaust emissions. This has encouraged technological development in many aspects of engine design. One way in which exhaust emissions are improved is to increase combustion efficiency by providing a fuel injector that is able to inject an injection pulse sequence comprising a pattern of discrete injection pulses during each combustion cycle. For example, a fuel injector may be required to deliver a relatively small volume of fuel in one or more so-called ‘pilot’ injections, followed by a larger delivery of fuel in a main injection event, followed by a relatively small delivery of fuel in one or more so-called ‘post’ injections.
A particularly advantageous system having the above functionality is a so-called ‘hybrid’ fuel injection system that combines known common rail technology and known “unit-injector technology”.
Referring to FIG. 1, a hybrid fuel injection apparatus 2 includes a pumping arrangement 4 that is arranged to receive fuel from a fuel supply means via a pump supply passage 8 and to supply pressurised fuel to a fuel injector arrangement 10 via an injector supply passage 12.
The pumping arrangement 4 includes a reciprocable pumping plunger 14, an end of which is slidably received within a pumping chamber 16. The pumping plunger 14 is driven by a cam drive arrangement (not shown on FIG. 1) to perform an outward stroke during which fuel enters the pumping chamber 16 from the pump supply passage 8 and an inward stroke during which the pumping plunger 14 moves inwardly to reduce the volume of the pumping chamber 16 thus increasing the pressure of the fuel trapped within it.
The pumping chamber 16 receives fuel via the pump supply passage 8 from one of two sources: i) a high pressure accumulator volume 20, or ‘common rail’, and ii) a low pressure transfer pump 22. Communication between the low pressure transfer pump 22 and the pump supply passage 8 is controlled via a pressure operable non-return valve 24 and communication between the common rail 20 and the pump supply passage 8 is controlled via an electrically operable two-way valve 26, hereafter ‘rail control valve’. The position of the rail control valve 26 determines whether the pumping chamber is supplied with fuel from the common rail 20 or the low pressure transfer pump 22.
The fuel injector arrangement comprises an injector nozzle body 30 defining a bore 32 within which a valve needle 34 is slidable. The nozzle body bore 30 is shaped to define an annular delivery chamber 36 surrounding approximately the mid point of the valve needle 34 and which communicates with the injector supply passage 12.
The valve needle 34 is shaped so as to define a relatively wide upper region 34a that tapers sharply approximately mid-way along the length of the valve needle 34 into a relatively narrow lower region 34b that terminates in a valve tip 34c. The valve needle tip 34c is enagageable with a valve seat 38 so as to open or close a nozzle outlet 40 and thus control the delivery of fuel from the injector arrangement 10. It should be mentioned at this point that although FIG. 1 shows a single nozzle outlet 40, this is for simplicity only and, in practice, such an injection nozzle is likely to include a plurality of nozzle outlets radially disposed about the tip of the nozzle.
The tapered region between the upper region 34a and the lower region 34b defines a thrust surface against which pressurised fuel acts to impart an opening force to the valve needle 34.
The lower region 34b of the valve needle 34 defines a clearance with the bore 32 such that pressurised fuel can flow from the delivery chamber 36, to the nozzle outlet 40 past the valve needle 34. The upper region 34a of the valve needle 34 defines a close sliding fit with the bore 32 so as to provide guidance to the valve needle 34.
The end of the valve needle 34 distal from the valve needle tip 34c includes a surface that is exposed to pressure within a control chamber 42. The pressure of fuel within the control chamber 42 is determined by the state of a three-way, two-position valve 50, hereafter the ‘nozzle control valve’. When the nozzle control valve 50 is in a first (closed) position, the control chamber 42 communicates with high pressure fuel within the injector supply passage 12. The high pressure within the control chamber 42 exerts a closing force on the valve needle 34 such that it remains engaged with the valve seat 38, thus preventing fuel delivery through the nozzle outlet 40. The control chamber 42 houses a biasing spring 43 that provides an additional closing force on the valve needle 34.
When the nozzle control valve 50 is in a second (open) position, the control chamber 42 communicates with a low pressure drain passage 52. In this position, the closing force on the valve needle 34 is reduced such that the force due to the fuel pressure within the delivery chamber 36 acting on the valve needle thrust surface is greater than the closing force. As a result, the valve needle 34 is urged away from the valve seat 38 so that fuel delivery is initiated.
One particularly beneficial aspect of the hybrid fuel injection system exemplified in FIG. 1 is that the injection arrangement 10 may be supplied with pressurised fuel for injection from either the pumping arrangement 4, at a relatively high pressure level, or from the common rail 20 at an intermediate pressure level, thus providing the flexibility to vary the pressure of fuel delivered to the combustion chamber.
In order to inject fuel at common rail pressure, the rail control valve 26 is set to its open position. Fuel at common rail pressure therefore flows from the common rail 20, though the pump supply passage 8, the pumping chamber 16 and the injector supply passage 12, to the injector arrangement 10. Opening the nozzle control valve 50 relieves pressure within the control chamber 42 such that the valve needle 34 is urged to lift from the valve seat 38 and so fuel injection is initiated. Closing the nozzle control valve 50 causes pressure to be re-established in the control chamber 42, thus urging the valve needle 34 to re-engage the valve seat 38.
In order to inject fuel at maximum pumping pressure, the rail control valve 26 is set to the closed position. As the pumping plunger 14 performs an outward stroke, pressure in the pumping chamber 16 reduces to a point at which the non-return valve 24 opens due to the pressure difference across it such that fuel at transfer pressure will flow from the transfer pump 22 to the pumping chamber 16. As the pumping plunger 14 begins its inward (pumping) stroke, the pressure of fuel rises so as to close the non-return valve 24 thus preventing pressurised fuel from flowing back to low pressure. High pressure fuel is therefore transmitted to the injector arrangement 10.
A high pumping pressure injection is initiated by opening the nozzle control valve 50 as the pumping plunger 14 approaches its innermost position (i.e. during a pumping stroke). Opening the nozzle control valve 50 relieves the pressure of fuel within the control chamber 42 which permits the valve needle 34 to lift from the valve seat 38, thus initiating an injection event.
As will be appreciated from the above description, the hybrid fuel injection system 2 provides considerable flexibility of injection timing and injection pressure. The availability of a substantially constant pressure fuel source from the common rail 20 permits injection events to be performed at any point in a combustion cycle. Moreover, the pumping arrangement 4 permits one or more high pressure injections of fuel driven from the pumping plunger 14 for a limited duration of the combustion cycle.
The fuel injection system in FIG. 1 is able to inject a sequence of several fuel injection pulses within a combustion cycle. FIG. 2 shows one such sequence comprising six fuel injection pulses: first and second pilot pulses 60, 62, first and second main injection pulses 64, 66, which are shown merged in FIG. 2, and first and second post injection pulses 68, 69.
Each fuel injection pulse in the sequence in FIG. 2 may take one of the following injection pulse types:    i) the fuel injection pulse is disabled,    ii) the fuel injection pulse is at common rail pressure,    iii) the fuel injection pulse is at pump pressure,    iv) the fuel injection pulse is at pump pressure and terminated by pressure spill,    v) the fuel injection pulse is at common rail pressure in HCCI mode (homogeneous charge compression ignition). In common rail HCCI mode, the fuel injection pulse is spilt into several small injections so as to achieve a greater degree of homogeneity of the fuel/air charge within the combustion chamber.
The number and type of fuel injection pulses making up a specific fuel injection pulse sequence are defined by the term ‘fuel injection mode’, the precise configuration of which is determined as contributing to efficient combustion conditions at a given engine operating region. For example, pilot fuel injection pulses are often utilised to reduce noise/vibrations at engine idle as they help to ensure a homogenous fuel/air mixture injection thus promoting an evenly distributed flame front around the combustion chamber. Furthermore, permitting two instances of main injection pulses is usually used at low speed when pump pressure cannot be maintained during a long main injection pulse. Therefore, by splitting the main injection and allowing pressure to build up in between for a short time, it is possible to have a longer injection at an higher average pressure. Post injection pulses typically are used for regenerating particulate filters installed within the exhaust system.
Referring again to FIG. 1, the fuel injection apparatus 2 is controlled by an engine control unit (ECU) 70 which has overall responsibility for the correct operation of all aspects of the engine and, in particular, is operable to control the injection arrangement 10 to operate in one of a plurality of fuel injection modes, depending on the prevailing operating condition of the engine.
The engine control unit 70 includes a fuel injection mode module 72, hereafter ‘FIM module’, which monitors a variety of engine parameters 74 (for example, engine speed, engine load, outside air temperature, accelerator pedal position) and calculates the appropriate fuel injection mode that suits the prevailing operating conditions of the engine.
In the known fuel injection apparatus 2 of FIG. 1, all possible fuel injection modes that are suitable for use with the fuel injection apparatus 2 are predefined during manufacture and a unique identifier in the form of a real number is allocated, in sequence, to each fuel injection mode. The FIM module 72 calculates the appropriate fuel injection mode in response to the prevailing operating condition of the engine and outputs the appropriate fuel injection mode identifier to a fuel injection equipment control module 76, hereafter ‘FIE control module’, which is responsible for controlling the nozzle control valve 50 and rail control valve 26.
The FIE control module 76 receives the fuel injection mode identifier as an input and configures injection characteristics for the injection pulse sequence, such as the volume of fuel to deliver during each discrete injection pulse within the pulse sequence, the timing of the injection pulse sequence within the combustion cycle and the temporal separation between the discrete injection pulses within the pulse sequence. The FIE control module 76 outputs first and second electrical control signals 80, 82 to the nozzle control valve 50 and the rail control valve 26, respectively, in order to control their operation and, thus, control the fuel injection apparatus 2.
In order to configure the appropriate settings for the injection pulse sequence, the FIE control module 76 uses the fuel injection mode identifier to reference a look up table 78 that is stored as a globally accessible data structure within the FIE control module 76.
The FIE control module 76 communicates with the look up table 78 by way of a data link 84, retrieves the injector pulse sequence that corresponds to the identifier that the FIE control module 76 received from the FIM module 72 and uses the injection pulse sequence data during subsequent calculations of the injection characteristics.
This above process repeats at a frequency of approximately 50 to 100 Hz whilst the engine is in operation in order to readily adapt to the changing operating condition of the engine.
The above described apparatus is disadvantageous in certain respects. Firstly, the operation of the FIE control module 76 as described above imposes a burden on the processing capacity of the engine control unit 70. Secondly, a significant memory resource of the ECU 70 is given over to the storage of the injection pulse sequences. Thirdly, it is problematic for engineers to perform system testing due to necessity to refer to look up tables in order to configure the fuel injection modes in test routines.
The inventors have devised a fuel injection apparatus that modifies the way in which the fuel injection mode is defined which reduces the processing requirement of the engine control unit since it can be manipulated more readily by software functions therein, and which may be interpreted by engineers intuitively without the need to reference data tables during system configuration and calibration.